Bug 1 (high): replace fixed sleep 1 with caller-PID polling in both
kickstart and start-after-exit handoff modes. The helper now waits until
kill -0 $caller_pid fails before issuing launchctl kickstart -k.
Bug 2 (medium): gate enable+bootstrap fallback on isLaunchctlNotLoaded().
Only attempt re-registration when kickstart -k fails because the job is
absent; all other kickstart failures now re-throw the original error.
Follows up on 3c0fd3dffe.
Fixes#43311, #43406, #43035, #43049